Arburg's egg-cellent example of LSR molding

Lossburg, Germany — Arburg GmbH + Co. KG featured three liquid silicone rubber demonstrations at its annual Technology Days open house.

All involved two-component molding, with two LSR/LSR applications and one with LSR/polycarbonate.

As was the case for Arburg's booth at K 2016 — when it molded two-color watch straps, followed by mounting of watch cases and metal buckles into the straps — the production cell was a major attraction.

The other LSR/LSR demonstration also involved two colors, in the form of two-color egg cups with the larger white component being complemented by the smaller secondary yellow core of the cup. These colors were chosen to harmonize with the egg white and yolk of a real hard-boiled egg.

The soft cups in Silopren 2670 from Momentive Performance Polymers were rigid enough to support an egg, and created a distinct impression of having similar consistency to real egg whites and yolks. In any event, LSR has more than adequate heat resistance to cope with boiled eggs!

Momentive's Silopren 2670, used for the egg cups and as one of the two LSR/LSR watch straps, has the same lower viscosity as the company's standard 2070 grade, with improved tear resistance and a higher reactivity, which enables lower vulcanization temperature at a standard cross-linking rate. The resulting high curing speed and ease of demolding benefits molding of large numbers of articles in short cycle times, according to Momentive.

Arburg molded the 15 gram egg cups with 25-second cycle time on an Allrounder 470A injection molding machine equipped with a size 170 injection unit that first injects the white LSR for four cups into the lower half of a 4+4-cavity mold. The mold core rotates 180 degrees to allow injection of the yellow secondary "yolk" LSR component in the upper mold half via another injection unit mounted to a flange on the mold.

Arburg pointed out, however, that this secondary injection unit is not fully integrated in the Arburg Selogica control system.

Oftering, Austria-based mold and LSR dosing system producer Elmet Elastomere Produktionsund Dienstleistungs GmbH produced the rotating core mold, which has an insulation plate to prevent premature LSR cure outside the mold. It also supplied its latest TOP 5000 P multi-component dosing system.

Elmet points out that its cold runner system nozzles form part of the mold cavity surface, so the gating point is hardly detectable. A long needle guide and a patented infinitely variable setting mechanism for the pneumatically powered opening stroke ensure low-wear and high precision. The way in which the LSR is supplied to the nozzle shaft and the shape of the nozzle-needle combination ensure lowest possible LSR shear, according to Elmet.

The LSR/polycarbonate demonstration involved overmolding with 60-second cycle time on an electric drive Allrounder 370 of a 14.6 gram LSR strip in Wacker Chemie's Elastosil 3071/40 onto a transparent PC panel in a 1+1-cavity mold produced by mold maker EDEGS Formenbau GmbH in Freiberg am Neckar, Germany. EDEGS developed its first cold runner system for LSR molding in 1986, and it still represents the company's core business today.

The company is proud of its VARIOmold modular mold tool change system that received patent protection in 2000.

2 Komponenten Maschinenbau GmbH (2KM) in Marienheide-Rodt, Germany provided its SilcoStar 924 metering, mixing and dosing unit to prepare and supply LSR for the injection molding machine. SilcoStar 924 features hydraulic drive pumps combined with a 2.8 kW servo-electric drive gear pump, with pressure control ensuring homogenous LSR density in injection molding machine screws.

In a poster shown at the March 2017 Silicon Elastomers conference in Würzburg, Germany, 2KM stated that the hybrid drive SilcoStar 924/904 units use 90 percent less energy than pneumatic units, as do hydraulic drive 922/902 and all-electric drive e-Flow models.

Michael Hartung of the Institute for Materials Technology at Kassel University showed the innovative way in which the PC is activated in the mold by short-wave (200-280nm) 9-Watt UV-C radiation applied within the mold area by the robotic handling gripper. This results in high adhesive bonding strength of around 2.25 N/mm between LSR and PC, through formation of functional groups on the thermoplastic surface.

Hartung's colleague Ralf-Urs Giesen presented further details on the process and results one week after the Arburg open house at the SKZ Silicon Elastomers conference in Würzburg.

In his introduction to the subject, Giesen described how this work has been undertaken in the "Unipace" facility at IWS. Unipace was founded by Kassel University and with pharmaceutical company B. Braun Melsungen AG in 2013, as a cooperative application development center activity conducting research.

This covers both LSR and HCR silicones as well as thermoplastics on equipment that includes four injection molding machines, three of which equipped for molding LSR. Unipace has eight scientific and technical staff, supported by six student assistants. A planetary dissolver has been added at the end of March, and Giesen said there were plans to soon start using software from Sigmasoft to simulate molding of LSR parts.

Giesen explained that although LSR bonds strongly to nylon and PBT, it requires special grades containing adhesion promotion additives to achieve LSR/PC bonding. Unipace has therefore sought to achieve LSR/PC combinations using standard LSR grades from Dow Corning, Wacker Chemie and Momentive. It has been doing this with UVC-C in-mold treatment that does not involve heat entering the thermoplastic and that also allows for partial bonding where required, by selectively desisting from treatment of some areas.

Giesen mentioned that alternative techniques known to work with LSR/PC include using an adhesive layer and treatments such as Plasamtreat's Openair low-pressure plasma, as well as corona and flame or silica coating treatments. The UV-C treatment can be applied as pre-treatment, with separate 9,000-hour lifetime UV lamps or with such lamps integrated in the linear robotic handling system or by grippers operating in the mold, the latter as shown at Arburg.

Although Unispace uses an EDEGS mold in its LSR/PC work, other equipment differs from that shown at the Arburg open house, namely use of LSR 200 ELA 200 and EMT dosing equipment from respectively German suppliers Reinhardt Technik GmbH in Kierspe and EMT Dosiertechnik (Efficient Metering Technology) in Werdohl.

The entire LSR/PC process, for which Unipace has applied for a patent, involves overall cycle time of 60 seconds, including around five seconds of pre-treatment within the mold area with the robotic handling system and with another five seconds needed to move the molded PC plate from one cavity to the other and 45 seconds for PC and LSR injection and LSR curing stages.

Based on work with LSR from Wacker Chemie and Makrolon brand PC from Covestro, it was found that while LSR bond strength to PC drops slightly after five second UV-C exposure and is at a similar level for LSR/nylon. Giesen showed a chart, however, in which the LSR/PA bond strength rose well above that for LSR/PC at 30 seconds, reaching around 3.25 N/mm. He concluded however that 5-10 second radiation time suffices to obtain good adhesion of LSR to PC and PC/ABS.

Giesen pointed to a particular advantage of the process: its low cost compared with other bonding processes such as plasma treatment. There are also no health risks beyond a 5-10 centimeters distance from the UV-C source, he stated. Adhesion has remain high after 12 months and UV-C treated PC granulate has also been found to enable high adhesion to LSR after it was sent for a trial at an external molding processor, Giesen said.